The rugged conditions of launching and of the environment in space include, amongst other things, high levels of acceleration and vibration, temperature extremes, and the transition between atmospheric pressure and the vacuum of space. Furthermore, every kilogram is very expensive to launch.
Thus, for structural elements, it is desirable for materials to be rigid and lightweight, presenting very good dimensional stability and thus little thermal expansion, associated with good ability to withstand any mechanical deformation that might be due to vibration or acceleration, and good behavior in the vacuum of space (little degassing, etc.).
In certain applications, a structure in space needs to withstand and contribute to maintaining a large temperature difference: it is then necessary to have a material that is thermally insulating. An example of such an application concerns containers for cryogenic substances, where the temperature difference across the structure can be of the order of several hundreds of degrees.
A material that is commonly used for space applications is honeycomb material. Honeycomb material does indeed resemble the structure built by bees in a hive, i.e. a plane array of cells that are substantially hexagonal having walls constituting partitions between adjacent cells.
Honeycomb structures are light in weight and can be made very rigid relative to their weight. They can be stiffened, for example by multilayer lamination, possibly having plane “skins” which are stuck to the end edges of the cell walls, thereby closing the cells.
In order to make such an insulating structure, it is known to improve insulation by providing protection in insulating layers, known in the art as multilayer insulation (MLI).
Putting MLI into place is a difficult manual operation which requires a large amount of cutting out. It is difficult to put MLI in hidden places. In addition, small radii of curvature degrade its qualities. Cuts in the MLI, its positioning by means of Velcro with possible sagging, and the fixing zones of the structure itself are all potential causes of heat leakage. Conductivity between the structure and its fixing point is large.
With that method, the two functions of rigidity and of insulation are separate.
Document D1=U.S. Pat. No. 5,230,914 discloses an insulating material having a honeycomb of paper which is designed for use in temperature insulated food packaging so as to maintain refrigeration temperatures during transport. The honeycomb is sandwiched between two skins of aluminized paper. According to the teaching of that document, the effect of the honeycomb is to reduce heat transfer through the insulating material by reducing convection between the two skins. That effect is obtained by keeping air captive inside cells that are as small as possible, thereby preventing the air from moving.
Although analogous in shape to the honeycomb materials already known for use in space, the honeycomb of document D1 is totally unsuitable for space applications for several reasons. Firstly the material from which it is made: paper or card are materials that are not sufficiently clean and that are not sufficiently strong for use in space flight. In addition, the looked-for effect comes from air being held captive in the cells, whereas in space there should no longer be any air therein. Without any inside air, the teaching of D1 can no longer operate.